This invention relates to thermoplastic foam production and a novel system for direct injection of volatile liquid blowing agent into a screw-type extruder. In particular, it relates to an apparatus for handling fluids and improvements in multi-component blowing agent injection.
In the manufacture of polystyrene foam sheet, rod, or other stock materials, significant advantages have been obtained in the use of systems for mixing molten resin with various normally gaseous or volatile liquid components under high pressure. Extrusion of the molten mixture into a low pressure zone results in foaming of the thermoplastic extrusion mass by vaporization of the volatile or gaseous components.
A variety of normally gaseous or liquid blowing agents have been proposed for olefinic or styrenic polymers, including virtually all of the common atmospheric gases and lower hydrocarbons. For polystyrene, the C.sub.4 -C.sub.6 alkanes have gained widespread acceptance, especially pentanes. Following a typical extrusion foaming step, the stock material is ordinarily aged before thermoforming into containers or the like. During aging the foam cells and polymeric matrix become partially depleted of volatile hydrocarbons, which enter the atmosphere, generally as non-toxic, odorless materials. However, potential atmospheric contamination by these by-products of foam manufacture has led workers to seek non-polluting alternative blowing agents, such as the usual atmospheric gases, e.g., nitrogen, carbon dioxide, etc.
Extreme volatility of most of these normally gaseous materials, however, has posed a considerable problem in controlling the foaming process, resulting in surface defects and/or corrugations in the sheet product. Despite numerous attempts to solve this problem, CO.sub.2 has not been successfully employed in continuous direct injection foam manufacturing. In order to overcome these problems, a system has been devised for injecting a mixture of alkane liquid and carbon dioxide liquid into a molten extrusion mass in a continuous extruder unit.